Essay on reinforcements in concrete industrial building.

PROJECT OF STRUCTURAL REINFORCEMENT IN INDUSTRIAL BUILDING OF CONCRETE

Sánchez Olivares, G. y Tomás Espín, A.

Departamento de Estructuras y Construcción

Campus de la Muralla del Mar. Universidad Politécnica de Cartagena

30202 Cartagena (Murcia)

Abstract

This paper describes a project structural reinforcement in an industrial building of reinforced concrete, needed for the change of use that would make the same to install more equipment and facilities as mentioned in the original project. To undertake the work, it was decided to form a team in which the integrated project management as a key aspect was adopted to achieve the objectives of time, economy and quality. As a general solution, the use of a reinforcement system consisting of laminated carbon fiber webs of high strength and tensile stiffness, coupled with the structural elements of reinforced concrete by means of a specific epoxy adhesive resin is proposedIn addition, another type of reinforcement is proposed to solve problems in some very localized areas of the building.

Introduction

This paper is based on a structural reinforcement project that required the Department of Structures and Construction of the Polytechnic University of Cartagena ( UPCT ) for a company located in the Valley of tailings Cartagena . This project was necessitated by a change of use in a building of reinforced concrete structure , which is needed to install more machinery and transport facilities as mentioned in the original project.

A fundamental aspect conditioned the project : time. Due to the need of the imminent installation of the new machinery, the deadlines were very small , both for the study and determination of the solution structural reinforcement , as for the realization of it .

To undertake the work, it was decided to form a research team at the Department of Structural and Construction UPCT , wherein the integrated project management was adopted as a key aspect in achieving the objectives of time, economy and quality. One of the crew took over the work of project manager in order to optimize human and material resources in order to reach the above goals.

As a general solution , the use is proposed , in beams and slabs of the structure of a reinforcement system consisting of laminated carbon fiber webs of high strength and tensile stiffness , attached directly to the concrete elements by means of a specific epoxy adhesive resin. In addition , another type of reinforcement is proposed to solve problems in some very localized areas of the building .With this solution, the results were satisfactory as this is economic , quality and, very importantly, they managed to reinforce the structure in a few weeks , meeting the deadlines established between the company and suppliers of machinery. Interestingly, the solution adopted for the reinforcement has the characteristic of flexibility against future changes in solicitations , allowing introducing new reinforcements, if needed , with minimal changes and reduced cost.

Planning and project development

Within the general objectives, deadline , cost and quality , other sub-objectives for which the following lines were set defined :

Study of the bearing capacity of the building: At this stage it was necessary to collect data from the original project and the company responsible for the construction of the building . Two models of analysis that the new situation of loads and holes in the floors , facilities for transportation and power was taken into account were defined. A model used the matrix formulation and the other model used the finite element formulation . The purpose of these models was to determine the bearing capacity of the building facing the new situation presented, as well as against the various hypotheses combination of actions. It is noteworthy that the parametric models were made in order to modify them easily in the event of unforeseen changes were introduced.

Study possible solutions and detailed definition of reinforcement: In order to select the most appropriate solution to the specific problem of the building was discussed and analyzed the different existing solutions today in order to achieve the main objectives of the project. The next phase , once reinforcement technique used consisted in developing the detailed plans necessary .

Monitoring the implementation of structural reinforcement: This phase was conducted in two sub-phases : strengthening beams ( not required reinforcing pillars) and reinforcement of slabs. Existed concurrency in the execution of the first subphase and defining in detail the reinforcement of slabs, to gain time . In this second subphase due to the procedures of the contractor who performed the structural reinforcement , changes were made in relation to the shape and size of some holes . Because of this, the model of finite element analysis was modified to redefine forged reinforcement solution . These control tasks were exercised during the three weeks it took to run the structural reinforcement (which had halved in the case of the plants had been without machinery).

Monitoring the implementation of the assembly of machinery and equipment: The purpose of this phase was to avoid the damage of the reinforcing elements and to monitor changes in the position of the equipment and facilities. In regard to the latter , also had to modify the model of finite element analysis to redefine the reinforcement solution . These control tasks were exercised during time needed for installation of machinery and equipment.

Phay comment that although the period of project life was short , there were many changes in the translation machinery (characteristics and position to fill in the floors ) as well as in the implementation of the gaps. The assembly of a flexible parametric model to changes and the decision to use the laminated carbon fiber allowing a very fast mount , with little preparation of resistant bands use elements , is a very useful technique when deadlines are short and reliable and durable solution is required . This decision had satisfactory results since the changes that appeared in the last two phases did not condition the delivery of the project or the cost or quality of reinforcement solution .

The solution adopted involved a relatively low cost if you consider that other techniques that require more runtime represent a lost opportunity to produce and earn profits in the range of difference between the execution time of the first and second . The costs of reinforcement would have been lower if they initially planned provision symmetrically assembled in the slabs and beams , since most of the armor in the bays where machinery was located initially distributed , regardless account a possible future change in location to areas where initially no equipment or installation is installed .

reinforcement methods

The strengthening of a structure can be a lot of procedures , some of the most common, the incorporation of steel , the addition of steel reinforcement , structural screed with or without armor, bonding precast concrete, shotcrete or supplemental steel sheets .

Apart from these conventional methods have been introduced in recent years the technique mentioned reinforcement , comprising supplementing tissue laminates and composites based carbon fibers, which are attached to the concrete by epoxy adhesive resins which act as longitudinal shear forces for transmitting . This technique starts to become a real alternative to the conventional reinforcement by bonding steel plates primarily in flexural reinforcements , so best specific mechanical properties and corrosion resistance , such as savings in the total process of reinforcement, up to 25% , thanks to the lightweight composite materials presented , facilitating their transport, handling and application , using light aids for short periods of time, which compensates for the higher price of the composites against steel .

DESIGN CRITERIA

As general recommendations on the design of reinforcement of structures, both compounds as more conventional materials materials, we can highlight the following : avoid the collapse of the structure in the case of removal of the reinforcement, as can happen in case of accidents or vandalism; see the previous state of deformation before reinforced structure to check for redistribution efforts at sectional structure ; and study the suitability of the reinforcement.

The general scheme of calculation would include checking the security level of the unreinforced structure , which must be greater or equal to one, the verification of the safety of the reinforced structure and verification of the United Service Limits .

To verify the security of the reinforced structure , it should be noted that the mechanical properties of the composites can vary over time due to phenomena such as the aging , causing it to have to apply severe restrictions on the operation voltages in respect of the short term . Consequently, to ensure an adequate level of security , you should limit the permissible operating voltage by reduction factors of the ultimate strength , which according to various codes and authors vary between 0.60 and 0.81 for the case of carbon .

Regarding the verification of serviceability limit states of cracking and deformation, although many empirical formulations exist in the literature , one can apply a simple and conservative approach for verification , such as the application of the criteria usually accepted for reinforced concrete adherents as mechanisms that develop between the steel inside the concrete are less effective than those experienced in the reinforcements through composite laminate , seen in the past a wide distributed and less cracking .

design modelBesides the usual actions own weight of the structure, using overloading , snow, wind and seismic actions referred to in the standard NBE AE -88 , was considered the exact distribution of the loads transmitted by the equipment, both existing and incorporating new , in each of the four floors. Then was used to calculate different scenarios combining actions. In Figure 2, the finite element model incorporating hollows and loads one of the slabs is shown .

The methodology for obtaining reinforcement of different structural elements of the building consisted of an analysis and design of all of it with the new calculation assumptions to check the security level of the structure and locate areas where placement is essential reinforcement . We consider three types of basic elements in building, solid slabs of different plants , pillars and beams. For sizing they all used the SPACE program CYPECAD v.99.1.b. Moreover, considering the slabs as the most critical of the structure elements to be in them where they were going to perform various drilling and location of the equipment , it was decided to analyze them more precisely using the finite element program ANSYS 5.6, to provide accurate data stress concentration locally in the slabs.

As for the sizing of the different structural elements , has adopted a reduction coefficient of the strength of concrete and steel 1.5 1.15 ; majorization and coefficients of permanent actions 1.5 and 1.6 overloads . For the study took into account current legislation in Spain , as is the basic standard NBE -AE 88 actions, the construction standard NCSE -94 seismic and structural concrete instruction EHE . It also took into account the previous statement concrete EH- 91 to be projected with this regulatory structure .

Forged Solid Slab

By CYPECAD program , knowing at each node of the mesh bending moments in two directions and the torque , obtaining slabs base armor and the longitudinal reinforcement are checked necessary reinforcement and the transverse cutting and punching , as case, and compared with existing reinforcement arranged for the construction of the work, as exploded drawings of the original project with the information provided by the construction company responsible for the execution of the work.

Furthermore, each slab was modeled using finite elements and analyzed using the ANSYS program , for which was used a type shell elements 93 which introduces the modulus of elasticity of concrete used in making the tiles .

The alleged properties for the materials shown in Table 1. For the concrete used (HA- 25 ) , because gravity loads may act , with its maximum value, for long periods of time , have been adopted reduced modulus values longitudinal deformation assessment dependent deformations in first approximation.

Material properties (MPa)

Eh-91

Ehe

Instant Module longitudinal strain drying the concrete

30042

27264

Instant Module longitudinal strain drying the concrete (corrected)

20028

18176

Modulus of elasticity of steel

2,1 × 105

2,0 × 105

Characteristic strength of steel

510

500

Coefficient Poisson of concrete (adimensional)

0,2

0,2

Tabla 1. Properties of Materials

Stresses in the slabs under the most unfavorable combinations of actions are obtained. These stresses are compared to the last minute to produce a beam rate of 20 cm thick (slab thickness) by 20 cm wide (separation of the slab reinforcement), armed with a rod 12 Æ superiorly and other Æ 12 inferiorly. With this simplification the analysis model is provided and is a major safety factor to disregard collaboration resistant plate type mechanism but only the resistance of type beams.

In Figure 3 the principal stresses in the vertical direction (the paper) of one of the slabs are shown. It can be seen that the highest tensile stresses occur 10.47 MPa in the periphery of two slots and in the areas where the slab is stiffened by the presence of internal reinforcement beams flat reinforced concrete.

Brackets and Beams

Similarly to the previous case , the supports in obtaining biaxial bending - compression reinforced each were tested . It was found that the pillars did not need additional reinforcement to be largely oversized and with sufficient security support the new state charges. As for the beams , the longitudinal reinforcement theoretically required for bending and torsion , as well as the transverse reinforcement was obtained . In this case, it was necessary to apply some of the reinforcement in the beams did not have an acceptable level of safety.

Solution structural reinforcement PROJECTEDOf the study performed was considered as the most suitable solution for structural reinforcement is described below.

In beams and slabs , the areas where reinforcement system formed by a rolled strip of high tensile glued to the concrete surface by means of an adhesive epoxy resin to be used is specified. The laminated strips are composed of a polymer matrix of epoxy resin reinforced with carbon fibers , and have a tensile strength of 2400 MPa and a modulus of elasticity of 1.5 × 105 MPa .

In some vain placing braces to a base metal profiles A42b steel with a provision in St Andrew's cross was proposed, such that the nodes of the pillars arriostren in his encounter with the concrete slab , getting thus greater intraslacionalidad the overall structure that impacts a better performance against buckling of the pillars .

The specific case of the circular hole of diameter 1.40 m in height to accommodate a plant +120.65 filter 55 kN , owing to the poor fit of the rolled strip reinforcements to such geometries , a booster is proposed by placing 3 metallic steel profiles HEB 120 A42b under it, two of them from beam to beam and third embrochalado to earlier .

conclusionsWe can say that the solution of structural reinforcement laminated carbon fiber bands is a reliable solution strengthening , which, on the characteristics of strength and rigidity to withstand unforeseen overloads in an initial structural design .

This solution has an essential characteristic : the ease and speed of placement. Therefore, it is especially useful for when deadlines are short reinforcement . It is also a generally economical solution , if we consider that the plant will be able to produce before if we use other more traditional solutions that require more time in their realization.

Of note is the need to define models of flexible analysis (such as parametric model used here ) that support effectively the changes, which most likely often occur , and which are in the implementation phase of the reinforcement, for various reasons .

Do not forget that the bands are sensitive to damage against concentrated on them ( placed on the slabs ) and before the fire , so to be protected properly, use being conditioned by industrial activity concerned pesos.